Splits the state_group_cache in two.

One half contains normal state events; the other contains member events.

The idea is that the lazyloading common case of: "I want a subset of member events plus all of the other state" can be accomplished efficiently by splitting the cache into two, and asking for "all events" from the non-members cache, and "just these keys" from the members cache.  This means we can avoid having to make DictionaryCache aware of these sort of complicated queries, whilst letting LL requests benefit from the caching.

Previously we were unable to sensibly use the caching and had to pull all state from the DB irrespective of the filtering, which made things slow.  Hopefully fixes https://github.com/matrix-org/synapse/issues/3720.

Bugfixes
--------

- Fix bug in v0.33.3rc1 which caused infinite loops and OOMs
([\#3723](https://github.com/matrix-org/synapse/issues/3723))

Don't log exceptions when failing to fetch server keys

LaterGauge needs to call thread safe functions

Not being able to resolve or connect to remote servers is an expected
error, so we shouldn't log at ERROR with stacktraces.

Fix exceptions when a connection is closed before we read the headers

This fixes bugs introduced in #3700, by making sure that we behave sanely
when an incoming connection is closed before the headers are read.

Make the logcontext filter not explode if it somehow ends up with a logcontext
of None, since that infinite-loops the whole logging system.

Use get_cache_factor_for function for `state_cache`

This allows the cache factor for `state_cache` to be individually
specified in the enviroment

Fix logging bug in EDU handling over replication

Logcontexts for replication command handlers

Fix logcontexts for running pushers

Use a producer to stream back responses

Refactor request logging code

add new error type ResourceLimit

Merge branch 'neilj/limit_exceeded_error' of github.com:matrix-org/synapse into neilj/limit_exceeded_error

Neilj/resource limit block event creation

Update the admin register documentation to return a real user ID